1. Field of Invention
This invention relates to a thin film magnetic device, and more particularly, to a thin film magnetic head.
1. Prior Art
with the salient advance of photolithography and thin film formation technology, thin film magnetic devices are currently considered promising in a variety of applications. For example, there is a demand for magnetic heads of higher performance which meet the requirements of high density magnetic recording. Compared to ferrite heads which are fabricated by machining a bulk material to form a magnetic pole, thin film magnetic heads which are fabricated by applying a fine processing technique are likely to meet such requirements and thus find rapid widespread use.
Magnetostriction is one of the important properties that must be met by the magnetic film forming the magnetic pole of a thin film magnetic head. There are two reasons. First, both magnetostriction and crystal magnetic anisotropy must be reduced in order to provide a high magnetic permeability. Magnetostriction of a reduced magnitude is then required.
The other reason is to reduce Barkhausen effect. Japanese Patent Publication (JP-B) No. 7401/1989 proposes, in conjunction with a thin film magnetic head having a magnetic pole of Permalloy, to use a magnetic thin film of a composition having negative magnetostriction, for example, a magnetic thin film of Permalloy with a Ni content of 81 to 84% by weight having a magnetostriction value of -1.times.10.sup.-6 to -5.times.10.sup.-6. Similarly, Japanese Patent Application Kokai (JP-A) No. 264620/1989 states that a magnetostriction value in the range of -1.times.10.sup.-6 to -1.times.10.sup.-5 is preferred and a Permalloy composition with a Ni content of 81 to 83% by weight is satisfactory to this end. As described in J. P. Lazzari et al., IEEE Transactions on Magnetics, MAG-7, 146 (1971), it is believed that the magnetization process should be a magnetization rotation mode rather than a magnetic wall transfer mode entailing Barkhausen noise; to this end, it is critical to impart uniaxial anisotropy having an axis of difficult magnetization in a magnetic path direction; and to this end, magnetostriction of a negative reduced value as mentioned above is necessary.
JP-A 180994/1989 discloses how to prepare a magnetic thin film of a uniform composition by taking into account the characteristics of a plating method. Since the composition range which is able to exhibit magnetostriction of a small negative value as proposed above is limited as narrow as (82 .+-.1) wt %, JP-A 180994/1989 is designed to establish a uniform composition distribution in the magnetic pole despite a current density distribution varying with a shape and other factors.
Also JP-A 76682/1993 discloses a thin film magnetic head comprising a magnetic pole having a magnetostriction distribution such that magnetostriction is opposite between peripheral and central portions. More specifically, the pattern peripheral portion has negative magnetostriction and the pattern central portion of a large surface area has positive magnetostriction. What is considered therein is only the stress acting on a planar pattern at the periphery. Then the pole tip portion has negative magnetostriction in a peripheral portion and positive magnetostriction in a central portion thereof. Additionally, both the upper and lower magnetic layers have the same magnetostriction distribution.
Journal of Japanese Applied Magnetism Society, Vol. 8, No. 2, 65 (1984) states that the anisotropy of a NiFe film formed on a step of polyimide resin is explained as stress induced anisotropy. It is uncertain whether the stress is compressive or tensile. It is described that a film of positive magneto-striction undesirably experiences a substantial change of the magnetization process at the step.
JP-A 264617/1989 discloses a thin film magnetic head wherein a gap layer or a stress relieving layer in the form of a non-magnetic metal layer is formed on an insulating layer for the purpose of mitigating the stress applied from the insulating layer to the upper magnetic pole for restraining any disturbance of a magnetic domain structure.
JP-A 195809/1992 discloses a thin film magnetic head including a magnetic core having an alloy composition which is different between a medium facing portion and a back contact peripheral portion. It is intended to vary the anisotropy constant of the alloy itself, with no reference being made to the magnetoelastic effect responsive to stresses. Then both the upper and lower magnetic poles are similarly constructed of an alloy composition which is different between a medium facing portion and a back contact peripheral portion.
As mentioned above, the prior art focused at providing induction magnetic anisotropy by forming a film in a magnetic applied field and paid attention to minimization of the disturbance-causing anisotropy resulting from the magnetoelastic effect, that is, to minimize magnetostriction within a negative value range and to minimize the magnetostriction distribution within the magnetic pole. All the stresses considered heretofore are tensile stresses while no consideration has been made to the compressive stresses applied from the organic resist layer during annealing after film formation. Then as a matter of course, the designers of thin film magnetic heads has paid no attention to the ratio of yoke portion width y to contact hole width x of the upper magnetic pole, y/x, which dictates the direction of a compressive stress applied to the upper magnetic pole. However, such techniques are difficult to manufacture thin film magnetic heads with minimal noise in a consistent manner.
Various thin film magnetic devices favor magnetic poles and magnetic cores which are configured to a ring shape capable of affording a closed magnetic circuit. In the manufacture of such thin film magnetic devices, it is necessary that a pattern including a multiplicity of fine magnetic film rings be formed on a substrate or wafer, and each fine magnetic film ring be given a magnetic path in its circumferential direction. To this end, it is necessary to impart anisotropy having an axis of difficult magnetization in a circumferential direction. This cannot be achieved by any available means, with a lowering of manufacturing efficiency.